Despite rapid progress in understanding the genetic and molecular basis of CFTR cause clinical disease. To obtain such knowledge we must focus on airway epithelia, because that is currently the site of the most significant disease. In CF airway epithelia, two electrolyte transport defects have been described: a lack of CAMP-regulated C1- permeability and an increased rate of Na= absorption. The former can be attributed to dysfunction of CFTR -- a phosphorylation -regulated C1- channel located in the apical membrane of airway epithelia -- but the mechanisms that generate increased Na+ absorption are unknown. Because electrolyte transport is abnormal in CF airway epithelia, it has been assumed that the quantity and composition of CF respiratory tract fluid will be abnormal. Consequently, many investigators have supposed that the pathogenesis and pathophysiology of CF lung disease results from abnormal respiratory tract fluid. That assumption, in turn, has lead to the development of a number of therapies directed at correcting assumed abnormalities of respiratory tract fluid. Unfortunately, the initial assumption has not been tested; that is the goal of this proposal. The first aim is to test the hypothesis that the quantity and composition of fluid generated by CF epithelia is abnormal. We will test this hypothesis by measuring the direction, rate, and regulation of fluid transport a nd the composition of the mucosal fluid in primary cultures of normal and CF airway epithelia. We will also ask whether a variety of therapeutic maneuvers alter the mucosal fluid. The second aim is to test the hypothesis that Na+ absorption is increased in CF airway epithelium as a result of the loss of CFTR anion channels. We will test the possibility that HCO3- leaving the cell through apical membrane CFTR anion channels acidifies the apical region of the cell. Based on studies in renal epithelia it is possible that a reduction in Ph inhibits Na+ channels. In CF epithelia the loss of a HCO3- conduction pathway in the apical membrane could remove the inhibition of Na+ channels. In CF epithelia the loss of a HCP3- conduction pathway in the apical membrane could remove the inhibition of Na+ channels, thereby increasing the net rate of Na+ absorption in CF epithelia. These studies should provide information that is central to understanding how CF alters the respiratory tract fluid and thus may give new insights into how mutations in CFTR cause disease.